1. Field of the Invention
This invention relates to temperature-responsive control means for controlling the electric power supplied to an electric heater and, more particularly, to a temperature-responsive control means for supplying turn-on pulses to a semiconductor controlled heating circuit only when a sensed temperature is within a desired range.
2. Description of the Prior Art
In heater applications, it is desirable that protection be provided against overtemperatures substantially above a desired temperature level. Accordingly, the prior art includes many control systems which utilize sensing means for sensing the temperature of the heater itself or the article being heated, such as water in an electric teakettle, and means for switching off the electric power input to the heater upon the sensing of such an overheated condition. Thereafter when the temperature being sensed drops below the critical level, some prior art control arrangements automatically withhold the further application of electric power until an operator indicates in some fashion, such as through the operation of a manual switch, that the reapplication of power is desired. In many applications such as the previously mentioned electric teakettle, this latter approach is desired since repetitive overheating will only result unless some corrective step is taken. For example in an electric teakettle, overheating will often occur simply because the teakettle has become dry and, under such conditions, it is clearly undesirable that electric power be reapplied before the teakettle is filled again.
In the past, control systems have also been provided for maintaining a desired temperature through the continued on and off cycling of the applied power. In such cases, the sensing of a temperature above the desired level calls for the switching off of the electric power input, but not the continued withholding of power once the sensed temperature drops below the desired level. Various forms of control systems have been used for maintaining a desired temperature, including pulse-generating means for supplying turn-on pulses to a semiconductor switch in series with the heater. These devices have included zero-voltage switching means for supplying the turn-on pulses at the zero-voltage crossing points of the applied electric power. The zero-voltage switching means commercially available include integrated circuits RCA-CA 3058, CA 3059 and CA 3079 produced by RCA Solid State Division, Somerville, N.J. 08876.